gubbins: add aarch64/arm64 builds

[martin-g]

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[martin-g]

The OSX ARM64 build fails with:

14:59:02 BIOCONDA INFO (ERR) The following packages are incompatible
14:59:02 BIOCONDA INFO (ERR) ├─ raxml-ng >1.0.1*  does not exist (perhaps a typo or a missing channel);
14:59:02 BIOCONDA INFO (ERR) └─ ska2 >=0.3.0  does not exist (perhaps a typo or a missing channel).